Intensity-Modulated Radiation Therapy
Intensity-modulated radiotherapy (IMRT) is a new form of three-dimensional conformal radiotherapy. IMRT is designed to address a major limitation of conventionally delivered radiotherapy: its inability to restrict the treatment beam to the tumor-bearing tissue. Theoretically, the impact of radiotherapy on many cancers would be far greater if it were possible to deliver the radiation so that only the target, regardless of its shape, received a lethal dose. This hypothesis provides the principal motivation for IMRT—that the delivery of a high radiation dose should be confined to a spatial distribution that conforms as tightly as possible to the spatial distribution of cancer cells. IMRT also can be used to selectively exclude the irradiation of radiation sensitive regions—such as the salivary glands, optic pathways, brain stem and spinal cord—which should permit increases in tumor dose to levels beyond those feasible with conventional external beam radiation therapy.
Figure 1:Intensity-modulated treatment plan showing a degree of dose-to-target conformation not possible with conventionally delivered radiotherapy
Two recent advances that make the clinical implementation of IMRT a reality are the development of inverse treatment planning algorithms and the dynamic multileaf collimator. Unlike conventional treatment, with IMRT the intensity of radiation is made to vary across the beam. To a first approximation, the intensity is roughly proportional to the target thickness along the beam as assessed from the beam's-eye-view. Where the target is "thickest," the beam intensity is at its greatest; where the target is thinnest, at it lowest. Since multiple beams are used, calculation of intensities across any one beam must consider the contribution of from all other beams. Determining the optimal intensity-modulation, therefore, is a difficult problem. It is sometimes referred to as the "inverse problem" for radiotherapy treatment planning. Inverse planning algorithms provide solutions to the problem. To clinically deliver an intensity-modulated beam, a dynamic multileaf collimator is used to sweep opposing pairs of tungsten leaves across the field. Modulation is achieved by varying the size of the gap between the leaves as well as the length of time the gap remains open at each location in the beam.
Figure 2: Beam's-eye-view image showing target volume in red and spinal cord in blue
Figure 3: Map of intensities calculated by inverse planning software for the beam shown in Fig. 2
Figure 4: Intensity-modulated beam delivered using a dynamic multileaf collimator and recorded on film
In January 1998, the Department purchased and installed the Corvus inverse treatment planning system (Nomos Corp.). Throughout 1998, Scott Johnson, Ph.D., Franca Kuchnir, Ph.D., and Chester Reft, Ph.D. completed an extensive series of experiments to verify that the University of Chicago treatment beams had been modeled correctly and quantify treatment planning uncertainties. Dr Johnson also led the development of quality assurance procedures IMRT, Department protocols for IMRT, high-precision immobilization devices, and on-line correction strategies that allow patients to be positioned for treatment quickly and with high precision. It is worth noting that the benefits of 3D conformal radiotherapy will not be realized if patients are incorrectly positioned for treatment.
In December 1998, we treated our first patient using IMRT. In the first half of 1999, nine patients were added to the list, including patients with cancer of the pancreas, brain, head and neck, and scalp. By the end of August 1999, the total number of patients treated with IMRT increased to twenty. Preliminary results indicate that the treatments are extremely well tolerated, with few of the side effects of conventional radiotherapy, e.g., severe speech and swallowing complications in head and neck patients. As we continue to accrue patients, a quantitative analysis of the benefits of IMRT is underway.
